The documentary “Gleason,” a big Sundance hit, is difficult to watch – and that’s the point. It’s about Steve Gleason, the NFL standout with the New Orleans Saints who, three years after his retirement in 2008, was diagnosed with ALS (Lou Gehrig’s disease). Six weeks later, his wife, Michel, discovered she was pregnant. Because Gleason was told that his ability to speak would imminently be lost, he began filming video diaries addressed to his unborn child, his first. These videos are the film’s emotional core. Director Clay Tweel utilizes home movies and football game footage of Gleason at various points in his pre-diagnosis life, but we didn’t even need these to highlight his physical decline. (He eventually speaks through an eye-triggered speech synthesizer.)

As a portrait of a strong marriage tested to the max, as well as a dissection of Gleason’s fraught relationship with his domineering father, “Gleason” leaves a lot for us to fill in. In the case of Steve and Michel, who gives birth to a bouncy baby boy, the saddest moment comes when he types out to her on his screen, “I’m wearing you down to bones.” But it’s not all sadness. Gleason, a fighter on the field, and who is still alive, spearheaded treatment for others diagnosed with ALS. That’s a big part of his story, too. Grade: B (Rated R for language.)